CHATFIELD STATE PARK — Federal water authorities are preparing to nearly double the amount of water in Chatfield Reservoir, raising the level by 12 feet and flooding state-run beaches and swimming, picnic and boating areas in the process.

The $100 million project, intended to help sustain new growth in water-poor Denver suburbs, also will inundate 45 acres of cottonwood, willow and Russian olive groves in the park, destroying habitat for about 60 bird species.

This has put the Audubon Society, Denver Field Ornithologists and other birder groups at the front of opposition to the project.

“You take away all those trees, all those birds that nested in the trees aren’t going to be here,” said Joey Kellner, who coordinates Denver’s annual bird count.

U.S. Army Corps of Engineers project manager Gwyn Jarrett acknowledged the impacts of expanding the 1,450-surface-acre reservoir, constructed to control flooding on the South Platte River. A delayed 2,000-page environmental-impact study remains under internal review.

“Mitigation” options in the works include planting new cottonwoods — “a compensation of up to 42.5 acres of mature cottonwood bird habitat.”

At least 10 of those acres would be outside the 5,600-acre Chatfield State Park. The total number of trees planted, and whether trees would be watered to ensure survival, are not specified.

“We want to make sure that the ecological benefits are captured — because there are losses,” Jarrett said. “We’re going to do as much on-site as possible. It’s imperative that the public be involved. It’s in (suburban water agencies’) interest to meet the environmental concerns, because that means less opposition.”

Peering into a spotting scope as he stood on a spit that juts into the water along the south shore of the reservoir, Kellner said species such as nuthatches, black-capped chickadees, downy woodpeckers, northern flickers and others that rely on woods may decline if the reservoir is expanded.

“It’ll be sad not to have mergansers here anymore. This is one of the only places in Colorado where they breed,” Kellner said. “Who knows where they will go?”

Fifty years ago, the South Platte River, relatively feeble to begin with, flooded occasionally as mountain snowmelt waters cut down through Denver to the prairie.

A 1965 flood swept away homes and businesses, killing 13 people and causing $300 million in damage. Chatfield Dam, built in 1979 for $85 million, was meant to tame the river as Denver developed.

Today authorities say the dam is strong enough to hold an additional 20,600 acre-feet trapped from the South Platte and Plum Creek. That would bring the total capacity to about 48,000 acre-feet of water.

The additional water could sustain about 41,200 households. (An acre-foot contains 325,851 gallons, enough to meet the needs of two families of four for a year.)

“The dam was built for much more flood storage than what is being held there now. There would be no additional dam construction and no sacrifice of safety,” Jarrett said of the expansion project, which gained momentum in the 1990s after the proposed Two Forks Dam was rejected.

“All the studies have shown that this increased water supply will not have any increased flood risk,” she said.

Colorado Water Conservation Board officials asked the Army Corps of Engineers to expand the reservoir on behalf of 15 metro water-user agencies.

Residents who pay monthly water bills would have to pay the $100 million if the federal engineers approve the project. The South Metro Water Supply Authority and water suppliers in Castle Rock, Centennial and other suburbs currently rely heavily on groundwater wells that in some areas are running dry.

State government projections indicate Denver-area water use in 2030 will exceed supplies by 22 percent.

A public-comment period on the project will last 60 days once the federal plan is released. The report will be released online, with copies of the document made available in public areas. Federal authorities say they’ll also hold public meetings.

At the reservoir, however, Kellner said many birders question whether authorities take the bird-count data they’ve supplied seriously.

A bald eagle floated by on a chunk of ice, scanning for fish, then fended off magpies before swooping west over Kellner’s head.

Most birders are not optimistic, he said, gesturing at the spit where he and others stood as the sun set. All this would disappear underwater, along with an adjacent boating marina.

“How do you fight the establishment?” he said. “When it comes down to people versus the environment, people usually win out.”

Bruce Finley covers environment issues, the land air and water struggles shaping Colorado and the West. Finley grew up in Colorado, graduated from Stanford, then earned masters degrees in international relations as a Fulbright scholar in Britain and in journalism at Northwestern. He is also a lawyer and previously handled international news with on-site reporting in 40 countries.

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